OP also do you have any drains in the basement you don’t use frequently? A shower, etc? Run some water in all the drains. Over time the water in the traps can dry out, and then you can get a sewer gas smell. I know you think you’ve pinpointed the location but gas can settle and human noses aren’t that good. It’s worth ruling it out because the fix is so easy.

If it’s a mouse, they’re so little. Wait a week, open the windows, and see if it is gone.

Thanks so much for the advice! I don’t have a cat so not sure how to locate it. But at least I know my options thanks to you all. This is so gross!! Have a good day!

Yup, dead mouse or rat. As another poster said, hope it's a mouse. You can try to cut a hole in the closet near the bottom where you think the smell is the worst (our cat alerted us to where our dead mouse was a few years ago) and then take it out. Otherwise you have to wait out the smell. It's awful. Buy a bunch of those charcoal air purifying bags and throw them in there. Open windows. Not much you can do.

OP here, no water pipes around the closet, and based on nasty smell, I think it’s a mouse (in gaithersburg so I don’t think we hsve rats). Do I just wait for it to decompose in the wall?! Or cut out the drywall and remove? Thanks again to you who have dealt with this!

It could also be a leak.

Yes. Sounds like a dead mouse in the wall.

dead mouse or dead rat.

hope for the dead mouse. smell will go away faster... it is amazing that such a small critter will smell so bad.

Hi all, there is a terrible dead animal (I think) smell in the closet underneath the stairs in my finished basement. This is in the center of the house. I see nothing. Has anyone experienced this? I will call the pest people today but if they can't see anything I'm not sure what they can do. Could it be in the unfinished part behind the closet wall? I have no idea how it could get there? There is no ductwork in the closet either. Any insight is appreciated!